1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for determining the geographical position of a mobile terminal within a cellular network, and specifically to accurately determining Time of Arrival values from timing advance values, which can then be utilized to estimate the location of the mobile terminal.
2. Background and Objects of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. A standardization group, European Telecommunications Standards Institute (ETSI), was established in 1982 to formulate the specifications for the Global System for Mobile Communication (GSM) digital mobile cellular radio systems.
With reference now to FIG. 1 of the drawings, there is illustrated a GSM Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) (terminal) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18. Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless.
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. The BTS is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible. It should be understood that the BSC 23 may be connected to several base transceiver stations 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If a MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that Mobile Station 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
The digital GSM system uses Time Division Multiple Access (TDMA) to handle radio traffic in each cell 22. TDMA divides each frequency (carrier) into eight time slots (physical channels). However, with other TDMA systems, more or less time slots can be used. For example, in the D-AMPS system, each frequency is divided into three time slots. Logical channels are then mapped onto these physical channels. Examples of logical channels include traffic (speech) channels (TCH) and Control Channels (CCH).
Pursuant to a recent Federal Communications Commission (FCC) Ruling and Order, cellular phone service providers within the United States must provide by October 2001 the capability to locate the position of a cellular phone making an emergency (911) call within the provider's system to within 125 meters with about 67% probability. Currently, as can be seen in FIG. 2 of the drawings, upon a network positioning request, the Base Station System (BSS) (220 and 240) serving the MS 200 generates positioning data, which is delivered to the Mobile Switching Center (MSC) 260. This positioning data is then forwarded to a Positioning Center (PC) 270 for calculation of the geographical location of the MS 200. The location of the MS 200 can then be sent to the application 280 within the network that requested the positioning.
In order to accurately determine the location of the MS 200, positioning data from each separate Base Transceiver Station (illustrated by 210, 220, and 230) within range is required. The minimum number of such positioning data from each BTS (210, 220 and 230) depends upon the positioning algorithm used, the accuracy desired, and other considerations. This positioning data for GSM systems can include a Timing Advance (TA) value, which corresponds to the amount of time in advance that the MS 200 must send a message in order for the BTS 220 to receive it in the time slot allocated to that MS 200.
It should be understood, however, that any estimate of distance can be used, instead of the TA value of GSM and other Time Division Multiple Access (TDMA) systems. When a message is sent from the MS 200 to the BTS 220, there is a propagation delay, which depends on the distance between the MS 200 and the BTS 220. This delay is commonly referred to as Time of Arrival (TOA). TOA is measured by the system with a certain error. Often, the TOA measurements are not available, therefore TA values can be used as an estimation of the TOA. TA values are expressed in bit periods, which, for GSM systems, can range from 0 to 63, with each bit period corresponding to approximately 550 meters of the distance between the MS 200 and the BTS 220.
Once a TA value is determined for one BTS 220, the distance between the MS 200 and that particular BTS 220 is known, but the actual location is not. If, for example, in the GSM system, the TA value equals one, the MS 200 could be anywhere in an annular region from a radius of 550 meters to a radius of 1100 meters. Two TA values from two BTSs, for example, BTSs 210 and 220, provide two possible regions where the MS 200 could be located (where the two annuluses intersect), which can be seen in FIG. 3 as regions 370 and 380. However, with three TA values from three BTSs, e.g., BTSs 210, 220, and 230, the location of the MS 200 can be uniquely determined with a certain degree of accuracy, as shown in FIG. 3 as region 370. Using an appropriate algorithm, with knowledge of the three TA values and site location data associated with each BTS (210, 220, and 230), the position of the mobile station 200 can unambiguously (except for certain geometrical configurations) be determined (with a certain accuracy) by the Positioning Center 270.
Therefore, Timing Advance (TA) values are obtained from the original (serving) BTS 220 and two or more neighboring (target) BTSs (210 and 230). By utilizing Timing Advance (TA) values, Automatic Location Identification (ALI) can be accomplished using currently-available mobile terminals and implementing relatively inexpensive modifications to the base station hardware and software. TA values may be used as an approximation for TOA (Time of Arrival--the transmit time from MS to BTS), which implies that any errors in the TA information are inherited by the triangulation algorithm used for locating the MS. For example, the BSS measures TOA with a certain standard error, and this measurement is used to assign the time slot. However, the TOA measurement error made by the BSS may be large enough such that the wrong TA time slot is assigned, i.e., the time slot does not bracket the true value of TOA. This will cause the accuracy of the algorithm using the TOA information extracted from the reported TA to deteriorate.
It is therefore an object of the invention to improve the accuracy of the Time of Arrival information extracted from the Timing Advance values reported by the Base Station Systems in order to more accurately determine the geographical location of a mobile terminal within a cellular network.
It is a further object of the invention to help comply with the recent Federal Communications Commission (FCC) Ruling and Order to locate the position of a cellular phone making an emergency (911) call within the provider's system to within 125 meters with about 67% probability.